1. Field of the Invention
The present invention relates to a photoelectric angle measuring apparatus for measuring the relative angular positions of two objects which are rotatable relative to each other. The apparatus includes a scanning unit with a scanning scale connected to one of the objects. The scanning unit scans an angle scale of a scale carrier which is connected to the other object. Corresponding graduations or scale marks of the scanning scale and the angle scale extend parallel to each other.
2. Description of the Related Art
Absolute or incremental angle measuring apparatus of the above-described type are particularly used in processing machines for measuring the relative positions of a tool relative to a workpiece to be processed.
In such angle measuring apparatus, the angle scale of a rotatable scale carrier which is connected to the rotatable object is scanned by a scanning unit fastened to a stationary object for obtaining measurement values for the relative angular position of the rotatable object to the stationary object.
These angle scales can be formed in two different ways. The angle scale may be formed on the surface of a circular graduated disk at the periphery thereof in the form of radial graduations or scale marks. On the other hand, the angle scale is formed on the circumferential surface of a cylindrical scale carrier in the form of parallel graduations or scale marks.
European patent No. 0,096,448 describes a photoelectric angle measuring apparatus in which a scanning unit scans in incident light a radial angle scale on the surface of a circular graduation disk at the periphery thereof. In this scanning unit, the light rays emitted from a light source are directed through a semi-transparent mirror, a condenser and an optically projecting element which includes a cylindrical lens whose focal line coincides with the axis of rotation of the graduated disk and a conical mirror, onto the radial angle scale of the graduated disk and are reflected by the radial angle scale through the elements mentioned above and an objective onto a parallel scanning scale of a photoelectric sensor unit. The optically projecting element composed of cylindrical lens and conical mirror serves for the coincidental projection of the radial angle scale onto the parallel scanning scale. This known angle measuring apparatus is of complicated construction.
In a printed publication published by the company Dr. Johannes Heidenhein GmbH entitled "Winkelmessgerat [Angle Measuring Apparatus] LIDA 360", July 1986, a photoelectric angle measuring apparatus is described in which an angle scale in the form of a steel tape is fastened on the outer circumferential surface of a cylindrical scale carrier and is scanned in incident light by a scanning unit.
Such a photoelectric sensing unit may include, for example, a light source, a condenser, a semi-transparent mirror, a plane sensing plate with a sensing scale and a photoelectric cell. The light rays emitted by the light source are deflected by the condenser to extend parallel, are directed perpendicularly toward the scanning scale of the plane scanning plate after passing the semi-transparent mirror, are reflected by the angle scale on the outer circumferential surface of the cylindrical scale carrier and through the scanning scale of the scanning plate toward the semi-transparent mirror and reach the photoelectric cell after being deflected by the semi-transparent mirror. The plane scanning plate extends perpendicularly relative to the optical axis of the ray path and parallel at a certain distance to the generatrix of the circumferential surface of the scale carrier. Because of the curvature of the circumferential surface of the scale carrier, only the light rays in the median plane through the optical axis of the ray path and the axis of rotation of the scale carrier are reflected by the angle scale of the scale carrier perpendicularly through the scanning scale of the scanning plate. Because of this curvature of the circumferential surface of the scale carrier, the parallel light rays outside of this median plane directed parallel to this plane onto the angle scale of the scale carrier are reflected at a corresponding reflection angle and, thus, no longer are perpendicularly directed through the scanning scale of the scanning plate. This increasing divergence of the light rays with increasing distance on both sides from the median plane negatively influences the quality of the scanning signal which is periodically generated by the photoelectric cell, so that the resulting measurement values for the angular positions of the two objects may be afflicted with measurement inaccuracies.
These measurement inaccuracies can be eliminated by reducing the curvature of the angle scale (i.e., increasing the radius of the scale carrier), by reducing the distance of the scanning scale of the scanning plate from the angle scale on the circumferential surface of the scale carrier, and by reducing the length of the scanning scale perpendicularly of the median plane. However, these three measures have limits. The radius of the scale carrier is predetermined by the type of application of the angle measuring apparatus and the distance of the scanning scale from the angle scale cannot be reduced beyond a certain limit because of reasons related to sensing technology. The length of the scanning scale may not be below a certain length because otherwise the maximum amplitude of the scanning signal produced by the photoelectric cell becomes too low for a problem-free formation of measuring values.
It is, therefore, the primary object of the present invention to improve the sensing unit of a photoelectric angle measuring apparatus of the above-described type in such a way that improved measurement values are obtained.